The present invention generally relates to an improved optical sensor and method for measuring the concentration of a chemical constituent in a biological fluid where measurement interference from other chemical constituents is present. More specifically, the invention relates to a system for measuring online a concentration of one or more analytes, such as creatinine and urea, in biological fluids, such as blood, body secretions, or fluids from clinical therapies, such as dialysis solution, (i.e. dialysate) and a method for using the same. The sensor measures optical absorbance of a fluid sample to determine a concentration of an analyte. Optical absorbance measurements may then be compared to optical absorbance measurements taken after a process that specifically targets and removes the analyte. Because the process removes the intrinsic absorbance of the analyte, a resulting change in optical absorbance may be used to measure the analyte concentration.
Creatinine is produced in muscle as a metabolic waste product and is present in serum, and other body fluids. Because serum creatinine concentration is inversely correlated to kidney function, creatinine measurement in serum and urine is one of the most common clinical tests ordered. Also, creatinine kinetics measured during renal therapies can be used to estimate solute removal efficiency and to estimate patient lean body mass—an index of patient malnutrition (Forbes G, and Bruining G J, Urinary creatinine excretion and lean body mass. Am J Clin Nutr 29: 1359, 1976; Keshaviah P R et al. Lean body mass estimation from creatinine kinetics, Journal of the American Society of Nephrology, 4, 7, 1994, pg. 1475–85).
Creatinine biosensors reported in the prior-art generally consist of two components: a chemical recognition component that targets only creatinine (with high specificity) and converts it into a measurable product, and a transducer component that detects and measures the product. The chemical recognition component may be, for example, biocatalytic (i.e. an enzyme), and the transducer may be, for example, electrochemical (e.g. amperometric, voltametric), or optical (absorbance, or fluorescence measurement) Examples of biosensors constructed using these technologies are available in literature (Killard A J, Smyth M R. Trends in biotechnology, 18(10), 2000, pg. 433–37).
The most common creatinine sensors are biocatalytic and based on measuring the products formed from one or more enzymatic reactions. Multi-enzyme biosensors (Tombach B. et al. Clinica Chimica Acta. 312(1–2):129–34, 2001; Rui C-S et al. Analytical biochemistry, 210 163–171, 1993) are more complex than single-enzyme biosensors because of their requirement of coupled reactions with enzymes and substrates. Prior-art single-enzyme catalysed reactions are based on the measurement of NH3 or NH4+ which is accomplished using either optical transduction (H. Li et al. Biosensors & Bioelectron. 7, 725–732, 1992) or electrochemical transduction (JP57074097 Measurement of creatinine and device therefore, 1982). However, these traditional single-enzyme biosensors are also disadvantageous because: 1) in addition to an enzyme, the optical measurement requires a separate (i.e. extrinsic) chemical indicator (calorimetric, fluorometric) for NH3 measurement that must be stable and accurate for the measurement duration (3 to 4 hours during dialysis); and 2) electrochemical transduction is invasive, prone to drift, and requires repeated sensor calibration.
A need, therefore, exists for a creatinine sensor that is less complex than the prior art and can be used for stable online measurements of creatinine in applications such as dialysis. Although the instrinsic ultraviolet (UV) absorbance of creatinine is well known (Adams W S et al. Analytical Chemistry, vol. 34, No. 7, 1962) a suitable biosensor that utilizes this intrinsic absorbance to measure creatinine in biological fluids has not been feasible because of the broad and overlapping absorbance spectra of many co-existing solutes. The applicants have found that a simple and accurate creatinine sensor can be constructed if absorbance measurements are combined with a process that specifically targets and removes creatinine.